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M. A. McGrath, E. Nelan (STScI), D. C. Black (LPI), G. Gatewood (U. Pitt.), I. Han (Kor.~Astr.~Obs.~Korea), S. Lubow, K. Noll (STScI), A. Schultz (CSC/STScI), T. F. Stepinski (LPI), T. Targett (Cardiff U. of Wales)
Doppler spectroscopy of {\rho1\,Cnc} has detected evidence of a companion with an orbital period of 14.65 days and a minimum mass of 0.88 Jupiter masses (Butler et al.~1997). Astrometric observations performed with the Hubble Space Telescope Fine Guidance Sensor 1r using a novel new observing technique have placed an upper limit on the astrometric reflex motion of {\rho1\,Cnc} in a time period of only one month. These observations, performed in April 2001, have detected no reflex motion induced by the 14.65 day period radial velocity companion, allowing us to place a 3\sigma upper limit of ~0.3\,milliarcseconds on the semi-major axis of this motion, ruling out the preliminary Hipparcos value of 1.15\,milliarcseconds (Han et al.~2001). The corresponding upper limit on the true mass of the companion is ~30\,{M\rm{J}}, confirming that it is a sub-stellar object. \newline
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skip=1pt\hangindent=2truepc\hangafter=1 } \refindent{Butler, R. P., Marcy, G. W., Williams, E., Hauser, H., and Shirts, P. 1997, ApJ, 474, L115--118} \refindent{Han, I., Black, D. C., and Gatewood, G. 2001, ApJ, 548, L57--L60}